In recent years, there has been an increasing demand for a lithium battery which is utilized as the primary power source of mobile terminals, mobile electronics, household small-sized power storage apparatus, and motor-driven vehicles such as two-wheel automatic cars and hybrid electric cars, or the like.
Since many of solid electrolytes used in lithium batteries contain combustible organic substances, ignition or the like may happen when abnormality occurs in batteries. Under such circumstances, ensuring safety of batteries has been demanded. In addition, in order to improve reliability against impact or oscillation, to further improve energy density, as well as to meet strong social requirements for establishing a highly efficient energy conversion system which does not contaminate the earth, development of all-solid lithium secondary batteries using a solid electrolyte composed of a non-combustible solid material is desired.
As the non-combustible solid electrolyte, a sulfide-based solid electrolyte has been studied. Examples of the production method thereof include a method in which a raw material is treated at high temperatures under vacuum or in an inert atmosphere and a method in which mechanical milling is conducted at room temperature by using a planetary ball mill. However, none of these methods is suited to mass production due to the need of special equipment.
To respond to these problems, the inventors have proposed a method in which a lithium ion conductive solid electrolyte can be produced advantageously on the industrial scale at a relatively low temperature without using special equipment by allowing raw materials to react in an organic solvent (see Patent Document 1). Specifically, N-methyl-2-pyrrolidone or the like is used as a non-protonic organic solvent, and lithium sulfide and phosphorous sulfide are reacted in the solvent as a homogenous solution.
However, among non-protonic organic solvents, solvents which have a relatively strong polarity such as N-methyl-2-pyrrolidone easily dissolve phosphorous sulfide, and hence they become a factor of increasing reactivity. However, due to the strong solvating power with lithium, they tend to remain in lithium sulfide products.
According to the technology of Patent Document 1, it is possible to realize prescribed ion conductivity by removing residual N-methyl-2-pyrrolidone almost perfectly. However, by this method, the prescribed ion conductivity cannot be attained satisfactorily. It has a problem that the production process is prolonged since a generated solid electrolyte is required to be washed by using a non-polar solvent or the solvent has to be distilled off a number of times under a reduced pressure.
Further, if a polar solvent such as N-methyl-2-pyrrolidone remains in a product when the solvent is distilled off by a normal method, significant lowering in ion conductivity occurs, and hence, improvement is needed to ensure stable product supply. In addition, if the temperature at which a polar solvent is distilled off is too high, ion conductivity of a solid electrolyte may be lowered due to a reaction with a solvent.
A residual polar solvent may cause lowering of battery performance and cell corrosion. Therefore, it is necessary to remove it as perfectly as possible. Further, since a special solvent such as N-methyl-2-pyrrolidone is expensive, a solvent which is not only effective but also inexpensive has been desired.
Patent Document 2 discloses a method for producing a solid electrolyte in which raw material mixed powder is treated at a relatively low temperature by means of special equipment such as a tumbling mill. This method suffers from problems that it needs special equipment and that production efficiency is lowered due to adhesion of raw material powder to the wall of an apparatus.
Patent Document 1: WO/2004/093099
Patent Document 2: JP-A-H11-144523
The object of the invention is to provide a method for producing a lithium ion conductive solid electrolyte easily without using special equipment.